Brown adipose tissue (BAT) plays a key role in the evolutionarily conserved mechanisms underlying energy homeostasis in mammals. It is characterized by fat vacuoles 5-10 microns in diameter and expression of uncoupling protein 1 (UCP1), central to the regulation of thermogenesis. In the human newborn, depots of BAT are typically grouped around the vasculature and solid organs. These depots maintain body temperature during cold exposure by warming the blood before its distribution to the periphery. They also ensure an optimal temperature for biochemical reactions within solid organs. BAT had been thought to involute throughout childhood and adolescence. Recent studies, however, have confirmed the presence of active brown adipose tissue in adult humans, with depots residing in cervical, supraclavicular, mediastinal, paravertebral and/or suprarenal regions, for example. Also, it has also been reported that BAT, alternatively called beige fat can be found within white adipose tissue (WAT).